1. Field of the Invention
The invention relates to a method of producing a planographic printing plate, and particularly to a method of producing a planographic printing plate providing good processability.
2. Description of the Related Art
In general, planographic printing plates have lipophilic image areas which receive ink during printing, and hydrophilic non-image areas which receive dampening water. Planography is a process of printing based on repellency between water and printing ink, in which lipophilic image areas of a planographic printing plate work as ink receptive areas, and hydrophilic non-image areas thereof work as dampening water receptive areas (non-ink receptive areas), whereby the ink unevenly adheres to the surface of the planographic printing plate, deposits on the image areas alone, and is transferred to a medium such as paper.
In order to make a planographic printing plate, a planographic printing plate precursor (PS plate) composed of a hydrophilic support having thereon a lipophilic photosensitive resin layer (e.g., a photosensitive layer, or an image recording layer) has been widely used. Usually, a planographic printing plate is obtained by plate-making including an exposure step in which a planographic printing plate precursor is subjected to image-wise exposure through an original image such as a lith film, and a development step in which unnecessary portions of the image recording layer are removed by dissolution in an alkaline developer or organic solvent with the portions to be image areas of the image recording layer remained, to expose the hydrophilic support surface to form non-image areas.
As described above, in the prior art process of making a planographic printing plate precursor, a development step must be carried out after exposure thereby removing unnecessary portions of the image recording layer by dissolution in a developer or the like. However, from the viewpoints of environment and safety, it is required that the alkaline aqueous solution used for development be an aqueous solution having a pH close to neutral, and the amount of waste water discharged in the development step be reduced. In recent years, in consideration of global environment, disposal of waste water discharged in wet processing is a growing concern in the whole industries. Therefore, solutions of the problems are increasingly desired.
In recent years, digitization techniques for electronically processing, accumulating, and outputting image information using computers have been widely used, and various kinds of new image output systems suitable for these digitization techniques are put into actual use. Along with this, the computer-to-plate (CTP) technique is receiving attention. According to the technique, digitized image information is carried by a highly convergent radiation such as laser light, and a planographic printing plate precursor is scanned by and exposed to the light, whereby a planographic printing plate is directly produced without using a lith film. In particular, image recording materials usable for infrared laser recording are increasingly used, because they can be handled in incandescent light (white light). Of these image recording materials, positive-working image recording materials are receiving attention, which contain an infrared ray absorbing dye having a light-to-heat conversion property, and a phenol resin which prevents dissolution into a developer.
Usually, in the positive-working image recording material, the infrared laser exposure and heat generated by a light-to-heat converting agent hinder the dissolution preventive effect of the exposed regions to improve the solubility of the image recording layer in the regions, and the regions are removed by the development step to make a planographic printing plate. In general, after the development, a washing treatment with water is carried out to remove extra portions of the alkaline developer, followed by gumming, and then the planographic printing plate is used for printing.
The development treatment is usually carried out in an automatic developing machine. However, when the dissolved image recording layer increases in the developer, it precipitates to form development wastes (development scums). When a considerable amount of development wastes is generated, the wastes may adhere to the planographic printing plate after plate making to cause image defects. In particular, image recording layers usable for infrared laser recording tend to generate development wastes because they contain a photothermal converting agent having a relatively high molecular weight.
Since the alkaline developer used in the development step preferably has a pH close to neutral from the viewpoint of environment, various attempts have been made for achieving this. For example, International Publication No. 2009/094120A1 proposes a plate making method including treating a planographic printing plate precursor, which has a positive-working image recording layer having a layered structure, using a developer having a pH of from 6 to 11. However, reduction of pH in the alkaline developer alone cannot prevent the problem regarding developability, or the increase of development wastes due to the deterioration of the solubility of the image recording layer after repeated plate making using an automatic developing machine. As a result of this, development wastes may adhere to the plate surface after plate making to cause image defects.
Further, in order to improve hydrophilicity of the non-image areas and protect the plate surface of a planographic printing plate, it is usually preferred that the plate surface after development and water washing be subjected to a hydrophilizing treatment, which is called a gumming treatment. However, since the gumming treatment is a wet process, the same problems of waste water as in the case of the development step arise.